Power demand in an electric system varies considerably within the day, peaking during daytime and early evening hours and being at its minimum during the night. An electric power generating system has to be designed so as to be capable of supplying demand at peak times, and thus, during off-peak hours, a considerable proportion of an electric system generating capacity is not utilized.
In order to better the economy of an electric system, it is generally desired to find some use for the excess capacity during off-peak periods. One such use is the storage of energy during off-peak hours to be then utilized during peak consumption times to increase the energy production capacity.
There are various means for storing energy for the later production of electricity including electric energy storage in electric batteries, pumping of water into an elevated reservoir for the subsequent utilization of this water to drive a turbine to produce electricity in peak hours, as well as a CAES system. In a CAES system, air is compressed by an electrically driven compressor using off-peak electricity and stored in a compressed air reservoir. During peak electricity consumption periods, the compressed air is liberated driving turbines which produce electricity.
CAES systems are known, for example, from U.S. Pat. Nos. 3,677,008, 4,281,256, 4,391,552, 4,849,648, 4,237,692, 4,593,202, 4,765,142, 4,936,098 and 4,872,307. CAES systems generally comprise one or more electrically-activated air compressors (usually low pressure compressors in series with high-pressure compressors), a compressed energy storage or reservoir, often underground caverns, and expanders adapted to utilize the compressed air energy during its expansion for the production of electric power.
CAES systems available to date were generally provided with various auxiliary units aimed to increase the efficiency of the system. During compression, the temperature of the air rises and such heating reduces the system's storage capacity. In order to avoid this problem, it was generally the practice to provide coolers design to decrease the temperature of the air egressing from the compressors. Additionally, in prior CAES systems, various energy boosters were introduced aimed at increasing the compressed air's capacity to drive the turbine. Such energy boosters were, for example, various heaters, combustors and the like utilizing premium fuel. Specific types of such boosters were thermal energy storage systems which accumulate heat produced during the compression stage to later heat the expanding air, thus increasing the air's capacity to drive the turbine.